The present invention relates to pulse generators and in particular to the selection and control of executable protocols of the implantable pulse generator.
Cardiac rhythm management devices are devices that treat disorders of cardiac rhythm and include implantable pulse generators such as pacemakers and implantable cardioverter/defibrillators that provide electrical stimulation to selected chambers of the heart. A pacemaker, for example, is an implantable pulse generator that paces the heart with timed pacing pulses. The most common condition for which pacemakers are used is in the treatment of bradycardia, where the ventricular rate is too slow. Atrio-ventricular conduction defects (i.e., AV block) that are permanent or intermittent and sick sinus syndrome represent the most common causes of bradycardia for which permanent pacing may be indicated. If functioning properly, the pacemaker makes up for the heart""s inability to pace itself at an appropriate rhythm in order to meet metabolic demand by enforcing a minimum heart rate.
Also included within the concept of cardiac rhythm is the degree to which the heart chambers contract in a coordinated manner during a cardiac cycle to result in the efficient pumping of blood. The heart has specialized conduction pathways in both the atria and the ventricles that enable the rapid conduction of excitation (i.e., depolarization) throughout the myocardium. These pathways conduct excitatory impulses from the sino-atrial node to the atrial myocardium, to the atrio-ventricular node, and thence to the ventricular myocardium to result in a coordinated contraction of both atria and both ventricles. This both synchronizes the contractions of the muscle fibers of each chamber and synchronizes the contraction of each atrium or ventricle with the contralateral atrium or ventricle. Without the synchronization afforded by the normally functioning specialized conduction pathways, the heart""s pumping efficiency is greatly diminished. Patients who exhibit pathology of these conduction pathways, such as bundle branch blocks, can thus suffer compromised cardiac output.
Heart failure is a clinical syndrome in which an abnormality of cardiac function causes cardiac output to fall below a level adequate to meet the metabolic demand of peripheral tissues and is usually referred to as congestive heart failure (CHF) due to the accompanying venous and pulmonary congestion. CHF can be due to a variety of etiologies with ischemic heart disease being the most common. Some CHF patients suffer from some degree of AV block or are chronotropically deficient such that their cardiac output can be improved with conventional bradycardia pacing. Such pacing, however, may result in some degree of uncoordination in atrial and/or ventricular contractions due to the way in which pacing excitation is spread throughout the myocardium. The resulting diminishment in cardiac output may be significant in a CHF patient whose cardiac output is already compromised. Intraventricular and/or interventricular conduction defects (e.g., bundle branch blocks) are also commonly found in CHF patients. In order to treat these problems, cardiac rhythm management devices have been developed which provide electrical pacing stimulation to one or both ventricles in an attempt to improve the coordination of ventricular contractions, termed cardiac resynchronization therapy.
The present invention is a method and system for optimizing the operating parameters of a cardiac rhythm management device such as an implantable pulse generator in which a plurality of optimization algorithms are available. Such operating parameters may include, for example, the programmed AV delay and, if biventricular pacing is allowed by the physical configuration of the device, which ventricles to pace and the offset between ventricular paces. Optimization algorithms usually represent methods for setting parameter values that have empirically been shown to be effective in improving the cardiac status of at least some patients. Such algorithms reach a decision based upon various inputs including the physical configuration of the device, variables measured by the device, and patient data otherwise collected. Parameter optimization algorithms usually do not, however, produce information that is helpful in selecting which among a plurality of available algorithms is the optimum one to use in a given situation. In accordance with the invention, an indication of the degree of ventricular asynchrony existing in the patient is used to select an optimization algorithm. Other factors that may influence the selection include the physical configuration of the device and whether a selected algorithm produces parameter settings that are within allowable ranges.
In an exemplary embodiment, two parameter optimization algorithms are available. One algorithm adjusts the operating parameters in a manner that maximizes cardiac output while the other adjusts the operating parameters so as to maximize myocardial contractile functim (i.e., the strength of systolic contractions). The former may be implemented by a pulse pressure optimization algorithm since systolic pulse pressure is directly related to cardiac output at a given heart rate. An indirect way of determining pulse pressures produced by an atrial-triggered ventricular pacing mode with particular parameter settings is to measure the intrinsic atrial heart rate produced by the patient""s baroreceptor reflex. The pulse pressure optimization algorithm may then recommend the best parameter settings as determined from a series of trials, where the settings may include AV delay and/or which chambers to pace. The other optimization algorithm is one that adjusts the AV delay based upon a measured intrinsic atrio-ventricular conduction time (e.g., a PR interval on an electrogram). Parameter settings produced by such an algorithm have been shown to maximize myocardial contractile functim as reflected by the rate of change of systolic pressure. In accordance with the invention, an indication of the degree of ventricular asynchrony exhibited by the patient is used to select between the two optimization algorithms in a given situation. The morphology of an intrinsic QRS complex, or its equivalent in an electrogram, can be used as one indication of ventricular asynchrony. The width of the QRS waveform, as determined either from a single representative sample or from an average of such samples, is indicative of any delays that exist in ventricular depolarization. In a particular embodiment, the pulse pressure optimization algorithm is used in preference to the contractility optimization algorithm when the QRS width indicates a relatively large degree of asynchrony.